Cellulose esters are well-known polymers that have found use in applications such as plastics, film and coatings. These types of esters have been used, for example, as film forming agents in solventborne coatings. Cellulose acetate butyrates (CABs), in particular, have been extensively investigated for use in coatings. Other known cellulose esters incorporate different functional groups, such as carboxylate functionalities, to alter the properties of the esters or to provide sites for further reaction and manipulation of the functional groups.
WO 2006/116367 describes cellulose mixed esters which have a high maximum degree of substitution and comprise acetyl groups as well as C3-C4 esters. They are said to be useful in coatings applications, for example as the major components in high solids or low VOC compositions. WO 2007/145955 describes that such cellulose mixed esters, having both acetyl and C3-C4 ester functionalities, can be used to improve properties such as the gloss of a coating composition.
U.S. Pat. No. 5,420,267 describes cellulose acetoacetate esters. These are mixed esters, comprising acyl groups and acetoacetyl groups. Edgar at al. report that a higher degree of acetoacetate substitution can lead to crosslinked films which exhibit improved solvent and water resistance and hardness [K J Edgar, C M Buchanan, J S Debenham, P A Rundquist, B D Seiler, M C Shelton, D Tindall, Prog. Polym. Sci. 26 (2001) 1605-1688].
A desirable feature for mixed cellulose esters which are to be used in coatings applications is that they can be formulated into a waterborne dispersion. However, examples of mixed cellulose esters having this property are uncommon as applied to coatings. U.S. Pat. No. 3,220,865 describes a mixed cellulosic ester (CAB) formulation where the CAB (10-27% w/w) is emulsified into a 20-40% w/w formulation in the application of a coating suitable for wood surfaces.
One of the challenges for coatings applications is to produce cellulose esters which have a glass transition temperature (Tg) that falls within an appropriate range. In coatings applications, a number of factors, such as polymer size, degree of crosslinking and the presence of additives such as plasticisers can affect the Tg of the coating. The Tg itself influences properties such as adhesion and drying speed of the coating.
A cellulose mixed ester which is to be used as the principal binder in coatings, without added plasticisers or coalescing solvents, would preferably have a Tg which allows for film formation to occur at ambient temperatures. However, cellulose esters having glass transition temperatures in this range have proved elusive. For example, the cellulose mixed esters described in WO 2006/116367 and U.S. Pat. No. 5,420,267 have glass transition temperatures that fall within the range 75.27° C. to 120.37° C. and 136° C. to 225° C., respectively.
Current methods for lowering the Tg, such as the addition of plasticisers or coalescing solvents, are not always desirable. This presents a problem, as, to date, no cellulose mixed ester is known which has a Tg that allows for film formation to occur at ambient temperatures without the addition of plasticisers or coalescing solvents.
However, the applicant has now found that chemical modification of cellulose esters can allow for control and manipulation of the Tg. Such chemical modification can be achieved if, for example, one has access to a suitable cellulose starting material which incorporates functional groups that can be readily derivatised. A levulinyl group is one such functional group.
WO 2007/094922 describes ester derivatives of levulinic acid which are said to be useful as plasticisers and/or coalescing solvents in polymer compositions. WO 2007/094922 also describes a method for lowering the glass transition temperature of a polymer composition by adding to it a levulinic acid ester derivative. Among the esters contemplated are those comprising a levulinyl group covalently bound to, inter alia, polysaccharides such as cellulose. This document describes the hydrolysis of corn fibre and the synthesis of levulinic acid ester derivatives of the polysaccharide and polyol residues of the hydrolysis. However, the document does not describe suitable cellulose mixed ester starting materials that could be employed in the synthesis of chemically modified cellulose esters.
The production of a mixed ester containing levulinic acid was also reported as a synthetic product of cellulose [Vladimirova, Gal'Braikh, Peker and Rogovin Polym. Sci. U.S.S.R. 7 (1964) 868-873]. While explicit experimental data was not reported, the methodology required the use of highly refined pre-treated cellulose starting materiel (viscose silk). In the inventor's hands, the reported reaction conditions with perchloric acid failed to produce a highly substituted mixed-ester cellulose derivative. Furthermore, the mixed ester produced by Vladimirova, when significantly more perchloric acid was used, was found to be unsuitable, have very poor solubility (especially on storage) and a high Tg (120° C.). Further, the methodology of Vladimirova has been found not to work with less refined cellulose or pre-substituted cellulose starting material and it is not possible to tune the molecular size. The work of Vladimirova et al. shows the difficulty in producing mixed esters suitable for use in coatings.
Another challenge of coatings applications utilising cellulose esters is to provide solubility characteristics and properties that prevent ready dispersion or emulsification of the polymer. A cellulose ester that has a further reaction site that is suitable for tuning solubility and emulsifying properties is highly desirable. For example, the opportunity to tune the acid-number of the polymer such that stable emulsification, stable dispersion or even water solubility may be conferred onto the polymer back-bone.
There is therefore a need for cellulose esters, e.g. cellulose mixed esters that can be used as starting materials for preparing a variety of cellulose mixed ester derivatives, such as those which have glass transition temperatures that fall within an appropriate range to allow for film formation to occur at ambient temperatures. There is also a need for processes for making such cellulose esters. It is therefore an object of the invention to provide novel cellulose mixed esters, or to at least provide a useful alternative.